1. Field of the Invention
This invention relates to vibrator seismic sources having feedback control, and more specifically to seismic sources having feedback control in which the fundamental signal in the feedback is compared with the pilot signal and the control signal adjusted accordingly, and in which the distortion in the feedback signal reduces the amplitude of the pilot signal and the control signal is reduced accordingly.
2. Description of the Prior Art
Prior art vibrators detect phase differences between the vibrator motion and a pilot signal. The phase of the control signal for controlling the vibrator is shifted in phase to coincide with that of the pilot signal.
In more recent times, vibrator seismic sources have included means for examining amplitude as well as phase. In these prior art systems, the drive signal is increased in amplitude to increase fundamental acoustic energy. However, often the system is driven further into a nonlinear region. Furthermore, there is an accompanying increase in the harmonic distortion. Systems which compare peak amplitude of the feedback signal to the pilot signal, or which monitor force to determine when a decoupling of the pad from the ground is imminent do not take into account the distorton present in the force imparted to the ground.
In U.S. Pat. No. 4,616,352, issued on Oct. 7, 1986 entitled "Vibrator Seismic Source Having Distortion Limiting Control System" and assigned to the assignee of this invention, the component parts of the force applied to the ground are treated. That is, the fundamental signal and the real time distortion are separately treated rather than depending upon the entire signal itself. By so doing, the real time distortion is not inadvertently increased by increasing the drive to the vibrator and any increase or decrease in drive, when distortion is not a significant factor, is closely controlled because of the use of the fundamental signal.
The present invention also deals with treating the component parts of the force applied to the ground. However, in a typical survey, the composition of the ground may change dramatically. This invention treats trends in distortion as well as the real time distortion to allow for the change in composition of the ground.